The subject-matter of the present invention is a process for the purification of aromatic polyamines obtained by hydrogenation of the corresponding aromatic polynitro compounds.
It is well known that the products resulting from the hydrogenation reaction of aromatic polynitro compounds comprise a mixture of the various isomers of the polyamino compounds, as well as volatile by-products and by-products with a high boiling point (also referred to (herein)below as heavy by-products).
Thus, it is clearly necessary, on the one hand, to separate the aromatic polyamino compounds from the by-products but, on the other hand, it can prove to be important to separate certain isomers of these polyamines from the others, as not all are equally advantageous in subsequent applications.
Such a situation is found during the synthesis of toluenediamine (TDA), which comprises two types of isomers, the ortho and meta isomers, of which only the two meta isomers are of advantage, essentially in the production of toluene diisocyanate. In addition, the reaction mixture resulting from the hydrogenation reaction comprises volatile by-products, such as toluidine, for example, as well as heavy by-products.
It is known in the art to treat the reaction mixture resulting from the hydrogenation reaction of the corresponding aromatic dinitro compounds by distillation, after having carried out preliminary stages of dehydration of the reaction mixture and optionally of removal of the solvent, if it had been present during the hydrogenation reaction. The volatile by-products as well as the ortho-TDA isomers (2,3-TDA, 3,4-TDA) are recovered at the top of this distillation column; the heavy by-products comprising meta-TDA are recovered at the column bottom. The amount of heavy by-products can represent from 0.1 to 2.5% by weight of the recovered mixture.
The mixture of heavy by-products and of meta-TDA is then treated so as to evaporate all the meta-TDA present. However, this process is the cause of a loss in meta-TDA, which is trapped in the heavy by-products. This amount can represent 25 to 200% by weight of the heavy compounds.
The problem which is therefore not solved in a completely satisfactory way by the conventional processes is therefore to separate the meta-TDA isomers from these heavy by-products while losing the minimum possible amounts of these isomers.
With the aim of limiting the losses of this compound, Patent Application WO 94/006752 has provided for the separation of the meta-TDA from the heavy by-products by distilling this mixture in the presence of a third solvent with a boiling point greater than 290xc2x0 C. While this process makes it possible to limit the losses of the desired product in the heavy by-products, it requires, however, the use of a third compound, which increases the cost, as well as the installation of new equipment.
In German Patent DE 196 08 443, the solution recommended consists of an arrangement comprising the conventional vacuum distillation column and evaporators. More particularly, the mixture of polyamines is introduced into the vacuum distillation column, from which column is recovered, at the top, a mixture of light by-products, as well as the isomers of ortho-TDA. At the column bottom, all the stream, comprising the meta-TDA as well as the heavy by-products, is conveyed into an evaporator, where it is evaporated. The evaporated fraction is condensed and stored or used in a subsequent chemical reaction. The non-evaporated fraction is mixed with a stream drawn off from the side in the upper half of the distillation column and then evaporated in its turn. The products remaining liquid are separated, in order to be destroyed, for example; as for the evaporated fraction, it is returned to the distillation column. An alternative form of this arrangement consists in feeding the distillation column not with the mixture of TDA and of by-products but with the evaporated fraction of this mixture. The non-evaporated fraction is treated in the same way as the non-evaporated stream collected after the evaporation of the stream recovered at the column bottom in the preceding alternative form. The purified meta-TDA is then recovered at the bottom of the distillation column.
This document discloses a process which makes it possible to decrease the losses of meta-TDA present in the heavy by-products without, however, eliminating them. However, this process is complex and requires the installation of many items of equipment.
The aim of the present invention is to provide a simple and efficient process for the purification of a mixture of aromatic polyamines which does not require the installation of numerous expensive devices, without loss of aromatic polyamines.
These aims and others are achieved by the present invention, the subject-matter of which is therefore a process for the purification of a mixture of aromatic polyamines which are obtained by hydrogenation of the corresponding aromatic polynitro compounds, which mixture comprises the various isomers of the said amines as well as reaction by-products. The process according to the invention consists in carrying out a distillation of the said mixture and in then carrying out the following operations:
a first stream, corresponding to a fraction or to all of the stream recovered at the bottom of the distillation column, is separated and is used in a chemical reaction (such as phosgenation, for example),
a second stream is separated corresponding:
(i) to a stream drawn off from the side in the gas phase, in the lower third of the said column, this stream representing at most 20% or more than 80% of the total volume (first and second stream); or
(ii) to a fraction of the first stream, this fraction representing 10 to 50% of the volume of the first stream, in order to treat it subsequently.
The process according to the present invention therefore makes it possible to obtain desired polyamine isomers (meta isomers, in the case of TDA) which are devoid of heavy by-products, without loss of polyamines. This is because the unpurified fraction (comprising the meta isomers and the heavy by-products) is advantageously recycled in a subsequent reaction, such as phosgenation.